As a casing material of a positive electrode used in the coin-type storage cell such as electric double-layer capacitor is used mainly a metal in case of using a non-aqueous electrolyte, and a conductive synthetic resin in case of using an aqueous electrolyte. As the non-aqueous electrolyte is usually used an electrolyte obtained by dissolving a solute such as tetraethylammonium tetrafluoride borate or the like in a thermally stable, polar organic solvent such as propylene carbonate, acetonitrile or the like. When the electric double layer capacitor using such an electrolyte is charged, a casing for the positive electrode is exposed to an electrochemically oxidizable environment or an environment promoting an elution reaction of a metal. The casing for positive electrode exposed to such an environment has problems that the rise of internal impedance or remarkable elution reaction is accompanied with the oxide or hydroxide of the metal to cause pitting corrosion or whole corrosion to there by bring about the damage or the leakage of internal electrolyte.
Under the above environment, aluminum or stainless steel (JP-B-62-62449) or austenite-ferrite dual phase stainless steel having an aluminum layer in its interior (JP-A-62-94908) or the like has hitherto been used as a casing for positive electrode made of a metal. However, when the metal casing for positive electrode made of such a material is charged at a high voltage of about 2.8 V, there is a problem that the corrosion such as pitting corrosion or the like is easily caused and hence the electric double-layer capacitor having a high reliability can not be obtained.
As to such a problem, there have hitherto been proposed an alloy steel containing 0.1-2.0 mass % of N, a high corrosion-resistant austenite-ferrite dual phase stainless steel, a high corrosion-resistant austenitic stainless steel (see JP-A-63-81914) and the like as a raw material for the casing. However, these casing materials for positive electrode are insufficient in the corrosion resistance as a casing material for positive electrode being subjected to charging at 2.8 V. In the equipments using the electric double-layer capacitor, the performances have recently been enhanced, whereby the withstand voltage of the electric double-layer capacitor may exceeds 2.8 V. In this case, it has been confirmed that the excellent corrosion resistance is further required in the casing for positive electrode made of the stainless steel or the alloy steel.